1. Field of the Invention
The present invention relates to an improvement in a method of preparing a propanol sol of silica by a process comprising replacing the aqueous medium in an aqueous silica sol with propanol, and is more specifically directed to a method of preparing efficiently a stable propanol sol of silica having a specific surface area of 25 to 550 m.sup.2 /g, said sol containing no aggregate or gel of silica and having an SiO.sub.2 concentration of 10 to 50% by weight and a residual content of a medium containing 7.5% by weight or less of water based on the SiO.sub.2 content and 90% by weight or more propanol, by a process comprising replacing the aqueous medium in an aqueous silica sol with propanol by an atmospheric distillation method.
2. Description of Prior Art
In the specification of U.S. Pat. No. 2,375,738 is disclosed a method of preparing an organo-aquasol of silica which comprises preparing an aquasol of silica having a pH of 0.5 to 8.5 by peptizing an aquagel of silica with caustic soda, and adding thereto a water-miscible organic solvent such as propyl alcohol. The specification also discloses a method of preparing an organosol of silica by a process comprising adding to an aquasol of silica a water-miscible organic solvent having a boiling point lower than that of water, thereafter adding benzene so as to form a medium of ternary constant boiling mixture and removing all or a part of the water from the mixture by boiling.
The specification of U.S. Pat. No. 2,433,776 discloses a method of preparing a stable and substantially anhydrous organosol, such as butyl alcohol or butyl acetate sol of silica. The method comprises forming first a propyl alcohol sol, adding thereto the butyl alcohol or butyl acetate and finally removing the propyl alcohol by distillation. The propyl alcohol sol is formed by a process comprising preparing an aquasol containing dissolved inorganic salt, thereafter adding propyl alcohol so as to precipitate the salt, followed by removal of the water from the sol by distillation, then removing the precipitate of the salt and completely removing the water by distillation.
U.S. Pat. No. 3,351,561 discloses a method of preparing an organosol of silica which comprises the steps of adding an organic water miscible hydrogen bonding agent which has a dipole moment of at least 3.0 Debye units to a salt-free aquasol of silica containing from 20% to 60% by weight of silica to form a reaction system, with the amount of hydrogen bonding agent being present in a mol ratio per mol of surface silanol groups present in the silica particles of the sol of from 10:1 to 1:1, adding under vacuum to the reaction system a water miscible monohydric alcohol in an amount of from 1 to 7 volumes per volume of water present in the salt-free aquasol of silica, the alcohol having a boiling point greater than 50.degree. C., maintaining the vacuum and heating the reactants under conditions whereby substantially all the water is removed from reaction system, releasing the vacuum, heating the water miscible alcohol of the formed organosol of silica containing 20-60% by weight of silica to its reflux temperature at ambient pressure and maintaining the temperature for at least 1/4 hour. U.S. Pat. No. 3,351,561 also discloses that the hydrogen bonding agent provides a protective solvent shell around the silica particles to protect the particles from agglomerating which is believed to cause subsequent gelation or precipitation.
U.S. Pat. No. 3,855,145 discloses a method of preparing an organosol of silica which comprises the steps of passing an aquasol of silica containing 20-60% colloidal particles of amorphous silica through a cation exchange resin in the hydrogen form and a strong base anion exchange resin in the hydroxide form to produce a single deionized sol and after a holding time of 12-24 hours passing the single deionized sol through a cation exchange resin and an anion exchange resin to produce a double deionized silica sol having an SiO.sub.2 content of from 50 to 70% by weight, adding said double deionized sol of silica to a reaction vessel containing a water-miscible organic alcohol, feeding additional water-miscible organic alcohol as a water alcohol azeotrope is removed by vacuum distillation at a temperature below about 100.degree. C., and recovering the organosol of silica. U.S. Pat. No. 3,855,145 also discloses that when the water content is above about 5% to about 25%, precipitation occurs at temperatures below 100.degree. C., and that the reason why precipitation does not occur during the vacuum distillation preparation of the organosols when the water content falls within this range is that the total time of the vacuum distillation is short.
Japanese Patent Publication No. Sho 43-4649 (4649/1968) discloses a method of preparing a methanol sol of silica comprising adding methanol to an aqueous silica sol having a molar ratio of 300 or more as SiO.sub.2 /Me.sub.2 O, wherein Me represents a monovalent cation, and distilling out the water together with the added methanol so as to cause substitution of the water in the sol for methanol, thereby to form a methanol sol in which 0.35 mol or less of water is contained to 1 mol of the SiO.sub.2 content in the sol.
Japanese Laid-open Patent Publication No. Sho 59-8614 (8614/1984) discloses a method of preparing a sol of silica dispersed in an organic solvent, comprising mixing an aqueous silica sol with an organic solvent such as, for example, isopropyl alcohol and then removing water from the sol by ultrafiltration, thereby to form, for example; an isopropyl alcohol sol which has an SiO.sub.2 concentration of 20.3% by weight and a water concentration of 1.8% by weight that is calculated as about 9% by weight to the SiO.sub.2 content in the sol.
The method of replacing water in an aqueous silica sol with isopropyl alcohol by ultrafiltration has a problem in that it requires a remarkably long time to obtain a propanol sol having an SiO.sub.2 concentration of 20 to 30% by weight and a water content of about 3% by weight, because when the water content in the sol reduces to about 5% by weight, viscosity of the sol increases and the speed of the ultrafiltration decreases remarkably.
In the method of replacing water in an aqueous sol with isopropyl alcohol by atmospheric distillation, an increase in viscosity at a water content of about 10% by weight or less in the sol occurs, and additionally aggregates or gels of silica often occur at this water content in the sol, resulting in deterioration of the product.
U.S. Pat. No. 3,351,561 discloses a step of preparing an organosol of silica which comprises stabilizing the sol by adding a hydrogen bonding agent. However, the recovered organosol of silica tends to contain a residue of the hydrogen bonding agent. Accordingly, the usefulness of the sols is limited since the residues are inconvenient for some specific uses. And it is not efficient, from a commercial standpoint, to require the step of heating the organosol of silica to its reflux temperature and maintaining such temperature in order to partially esterify the silanol particles in the sol. U.S. Pat. No. 3,855,145 discloses multiple steps of the double deionization which comprises double deionizing and a holding time of 12-24 hours. Thus, from a commercial standpoint, the method which requires multiple steps is not efficient. Furthermore, the above-mentioned methods require additional equipments consisting of vacuum units, highly efficient refrigerators, highly efficient heat-exchangers (condensers) and so on. Accordingly, from a commercial standpoint, these methods are not desirable.
U.S. Pat. No. 2,375,738 discloses a method of preparing an organosol of silica which comprises replacing an aqueous medium in an acidic aquasol of silica with merely propanol by an atmospheric distillation. When the medium in the sol has a water content of about 15% by weight or less on replacing the medium, precipitation occurs and an increase in viscosity in the sol occurs. Therefore, from an industrial standpoint, the method is not desirable to prepare a stable and substantially anhydrous propanol sol of silica. U.S. Pat. No. 2,433,776 discloses a sol of silica which comprises a low molecular weight polysilicic acid prepared by the treatment of an aqueous silicate solution with a suitable acid. As the low molecular weight polysilicic acid has ordinarily a mean diameter of less than 3 milimicrons and a specific surface area of more than 700 m.sup.2 /g, the sol of silica is not stable for supplying commercial product generally.